The invention relates to a method for monitoring the formation of nitrogen oxide in an oxidation catalytic converter arranged in an exhaust gas system of a vehicle. By means of an exhaust gas aftertreatment device, a content of nitrogen oxides in the exhaust gas is reduced by converting the nitrogen oxides, and the content of nitrogen oxides in the exhaust gas is measured. Furthermore, the invention relates to an exhaust gas system for a vehicle.
The nitrogen dioxide content in the exhaust gas which flows into an exhaust gas aftertreatment device which is designed to reduce the content of nitrogen oxides in the exhaust gas, for example an SCR catalytic converter, affects the activity of the exhaust gas aftertreatment device with regard to the conversion of the nitrogen oxides. In the SCR reaction (selective catalytic reduction) in the SCR catalytic converter, the selective catalytic reduction reaction of nitrogen oxides with ammonia to form nitrogen and water depends on the nitrogen dioxide content due to different reaction paths for nitrogen monoxide and nitrogen dioxide. Because of statutory regulations, for example of the US American environmental authorities, the formation of nitrogen dioxide at the oxidation catalytic converter must be monitored as part of an on-board diagnostics system.
US 2009/0158813 A1 describes a method for monitoring an oxidation catalytic converter with regard to the formation of nitrogen dioxide, in which sensors arranged upstream and downstream of the oxidation catalytic converter respond differently to nitrogen monoxide and nitrogen dioxide. In this way, the formation of nitrogen dioxide at the oxidation catalytic converter is to be inferred based on the sensor signals.
As, however, the nitrogen dioxide content cannot be measured directly by means of a sensor, this method is subject to a certain inaccuracy.
Possible ways of indirectly determining the nitrogen dioxide formation at the oxidation catalytic converter, for example by determining the activity of the oxidation catalytic converter with regard to oxidation based on exothermic monitoring, likewise provide only comparatively inaccurate results. The same applies to the estimation of the activity of an SCR catalytic converter with the help of nitrogen oxide sensors, from which a nitrogen dioxide concentration is to be estimated. A reliable diagnosis of the nitrogen dioxide formation at the oxidation catalytic converter is also impossible by this means.
The object of the present invention is therefore to create a method of the kind mentioned in the introduction and an exhaust gas system for carrying out the method which enables the formation of nitrogen dioxide to be monitored particularly reliably.
With the method according to the invention, first of all, the conversion of nitrogen oxides corresponding to a first exhaust gas volume flow through the oxidation catalytic converter is measured. The exhaust gas volume flow is then varied and the conversion of nitrogen oxides which changes with the variation of exhaust gas volume flow is measured. The formation of nitrogen dioxide at the oxidation catalytic converter is inferred based on the respective conversion of nitrogen oxides at the different exhaust gas volume flows through the oxidation catalytic converter. In doing so, a predetermined relationship between the conversion of the nitrogen oxides and a proportion of nitrogen dioxide in the nitrogen oxides in the exhaust gas is taken into account.
This is based on the knowledge that the formation of nitrogen dioxide at the oxidation catalytic converter responds very sensitively to a change in the space velocity at the oxidation catalytic converter. The space velocity is the ratio of exhaust gas volume flow and volume of the oxidation catalytic converter and is usually expressed in the units “per hour.” An increase in the space velocity causes a reduction in the formation of nitrogen dioxide and vice versa. Here, this dependency of the formation of nitrogen dioxide at the oxidation catalytic converter on the exhaust gas volume flow through the oxidation catalytic converter is used to carry out a credible, that is to say particularly reliable, monitoring or diagnosis of the formation of nitrogen dioxide. A change in the exhaust gas volume flow namely leads to a change in the formation of nitrogen dioxide at the oxidation catalytic converter. As a result of this, the conversion behavior of the exhaust gas aftertreatment device varies, and these changes in the conversion of nitrogen oxides in the exhaust gas aftertreatment device can be measured by means of sensors designed for measuring the content of nitrogen oxides in the exhaust gas.
The formation of nitrogen dioxide at the oxidation catalytic converter connected upstream of the exhaust gas aftertreatment device can be inferred particularly accurately and reliably based on a known relationship between its nitrogen oxide conversion behavior and the nitrogen dioxide to nitrogen oxide ratio (NO2/NOx) which is known for the particular type of exhaust gas aftertreatment device. The nitrogen dioxide formation activity of the oxidation catalytic converter can therefore be inferred by means of the dependency of the activity of the exhaust gas aftertreatment device with regard to the conversion of nitrogen oxides on the nitrogen dioxide content in the exhaust gas. This enables the formation of nitrogen dioxide at the oxidation catalytic converter to be monitored particularly reliably. Here, the exhaust gas aftertreatment device preferably comprises at least one exhaust gas treatment component in the form of an SCR catalytic converter which, in a selective catalytic reduction of nitrogen oxides with ammonia, is able to catalyze to form predominantly nitrogen under oxidizing conditions. In doing so, the exhaust gas aftertreatment device is arranged downstream of the oxidation catalytic converter.
In a particularly advantageous embodiment of the invention, the exhaust gas volume flow through the oxidation catalytic converter is varied by varying a proportion of a low-pressure exhaust gas recirculation rate of a total exhaust gas recirculation rate. The total exhaust gas recirculation rate is made up of a high-pressure exhaust gas recirculation rate and the low-pressure exhaust gas recirculation rate. With high-pressure exhaust gas recirculation, the recirculating exhaust gas is diverted out of the exhaust gas flow before reaching the oxidation catalytic converter and introduced into the inlet air downstream of a compressor which compresses the inlet air for an internal combustion engine of the vehicle. On the other hand, with low-pressure exhaust gas recirculation, a partial flow of the exhaust gas is diverted out of the exhaust gas flow downstream of the exhaust gas aftertreatment device and therefore also downstream of the oxidation catalytic converter and introduced into the inlet air upstream of the compressor.
If, for a certain total exhaust gas recirculation rate, the low-pressure exhaust gas recirculation rate is now increased, then the high-pressure exhaust gas recirculation rate reduces. This leads to less exhaust gas being diverted upstream of the oxidation catalytic converter. As a result, the exhaust gas volume flow through the oxidation catalytic converter increases. This occurs without a driver of the vehicle noticing this increase in the exhaust gas volume flow through the oxidation catalytic converter. The space velocity can therefore be varied very easily and reliably by varying the low-pressure exhaust gas recirculation rate, thus affecting the formation of nitrogen dioxide at the oxidation catalytic converter. The conversion of nitrogen oxides in the exhaust gas aftertreatment device, which is dependent thereon, can then be measured with the help of nitrogen oxide sensors. This enables the formation of nitrogen dioxide at the oxidation catalytic converter to be monitored unobtrusively and, at the same time, reliably when driving.
It has been shown to be further advantageous when an ageing state of the oxidation catalytic converter is inferred by comparing the determined formation of nitrogen dioxide at the oxidation catalytic converter with an expected formation of nitrogen dioxide for the respective exhaust gas volume flows. Namely, if the actual nitrogen dioxide formation does not correspond to the expected nitrogen dioxide formation, then this is to be assessed as an index for an ageing and therefore deterioration of the conversion rate of the oxidation catalytic converter. If the ageing state of the oxidation catalytic converter is associated with a particularly large reduction in the formation of nitrogen dioxide, then the oxidation catalytic converter can be recognized as being defective. Accordingly, the attention of a vehicle user can be drawn to the need to replace the oxidation catalytic converter.
Preferably, the conversion of nitrogen oxides is defined for the different exhaust gas volume flows, while a temperature is present at the exhaust gas aftertreatment device such as is established at the exhaust gas aftertreatment device during the thermal regeneration of a particulate filter arranged in the exhaust gas system. During regeneration of the particulate filter, particularly high temperatures, in particular temperatures of more than 500° C., are present, namely upstream of the exhaust gas aftertreatment device. At these temperatures, an exhaust gas aftertreatment device in the form of an SCR catalytic converter no longer has any capacity for storing ammonia. In addition, the equilibrium reaction at the oxidation catalytic converter:NO+½O2NO2 
is thermodynamically shifted almost completely towards nitrogen monoxide.
Setting up conditions such as those which exist during the thermal regeneration of the particulate filter therefore ensures that external factors, such as the proportion of nitrogen dioxide in the exhaust gas which is fed to the exhaust gas aftertreatment device and the loading of the exhaust gas aftertreatment device with ammonia, have no disruptive or adulterating affect on the activity of the exhaust gas aftertreatment device during conversion of the nitrogen oxides. The conversion of nitrogen oxides by means of the exhaust gas aftertreatment device can therefore be determined at a temperature of more than 500° C. on the intake side of the exhaust gas aftertreatment device without disruptive frame conditions in the exhaust gas due to stored ammonia or due to the nitrogen dioxide content.
When determining the conversion of the nitrogen oxides at the different exhaust gas volume flows, it is therefore favorable when conditions such as those which occur in the course of an active thermal regeneration of the particulate filter are present. However, it is of particular advantage when the particulate filter has actually been regenerated, that is to say when the conversion of the nitrogen oxides at the different exhaust gas volume flows is determined particularly directly following the thermal regeneration of the particulate filter. Namely, there is then no carbon black loading in the particulate filter, which could likewise have a disruptive effect on the activity of the exhaust gas aftertreatment device. The conversion of the nitrogen oxides by means of the exhaust gas aftertreatment device can also be determined during the thermal regeneration of the particulate filter, in that the nitrogen oxide content in the exhaust gas is measured at least after the exhaust gas aftertreatment device and the measured value compared with a nitrogen oxide content which is present before the exhaust gas aftertreatment device.
Here, it has been shown to be advantageous when an ammonia content in the exhaust gas before thermal regeneration or after thermal regeneration of the particulate filter is taken into account when measuring the conversion of nitrogen oxides. The nitrogen oxide conversion is namely dependent on the ratio of the ammonia contained in the exhaust gas to the nitrogen oxide contained in the exhaust gas. This ratio is also referred to as alpha. The quality of the evaluation can be improved by varying alpha, as the conversion of nitrogen oxides depends on alpha. This diagnosis can be carried out, particularly in advance of the thermal regeneration of the particulate filter, which is to say in the heating-up phase, and/or in the concluding cooling phase.
It has been shown to be further advantageous when a temperature at the exhaust gas aftertreatment device is taken into account when applying the predetermined relationship between the conversion of the nitrogen oxides and the proportion of nitrogen dioxide in the nitrogen oxides in the exhaust gas. The ratio of nitrogen dioxide and nitrogen oxide corresponding to the nitrogen oxide conversion behavior is namely dependent on the temperature of the exhaust gas aftertreatment device. Taking into account the temperature therefore enables the formation of nitrogen dioxide at the oxidation catalytic converter to be inferred particularly accurately and realistically.
Preferably, a temperature-dependent correlation curve, which takes into account the formation of nitrogen dioxide at the oxidation catalytic converter at the different exhaust gas volume flows, is determined. This correlation curve is compared with an expected curve. Comparing the curves enables the variation of the formation of nitrogen dioxide at the oxidation catalytic converter due to age to be inferred particularly well.
Finally, it has been shown to be advantageous when the conversion rates of the nitrogen oxides corresponding to the respective exhaust gas volume flow through the oxidation catalytic converter are measured in a driving mode in which substantially constant driving speeds and/or substantially constant loads of an internal combustion engine of the vehicle are present. Namely, this enables disruptive influences due to significantly varying driving speeds or significantly varying loads to be particularly extensively excluded. It is namely expedient to monitor the formation of nitrogen dioxide in quasi-steady-state operation of the internal combustion engine.
The exhaust gas system according to the invention for a vehicle comprises an oxidation catalytic converter and an exhaust gas aftertreatment device arranged downstream of the oxidation catalytic converter for reducing a content of nitrogen oxides in the exhaust gas by converting the nitrogen oxides. Furthermore, at least one measuring device for measuring the content of the nitrogen oxides in the exhaust gas is provided. Furthermore, the exhaust gas system comprises a control device which is designed to infer the formation of nitrogen dioxide at the oxidation catalytic converter based on a predetermined relationship between the conversion of the nitrogen oxides and a proportion of nitrogen dioxide in the nitrogen oxides in the exhaust gas. For this purpose, the control device is designed to process measured values from the at least one measuring device which specify the conversion of the nitrogen oxides corresponding to a first exhaust gas volume flow through the oxidation catalytic converter and the changing conversion of the nitrogen oxides with the variation of the exhaust gas volume flow at changed exhaust gas volume flow. A particularly reliable monitoring of the formation of nitrogen dioxide at the oxidation catalytic converter can be achieved with such an exhaust gas system.
The advantages and preferred embodiments described for the method according to the invention also apply to the exhaust gas system according to the invention and vice versa.
The characteristics and combinations of characteristics stated above in the description and the characteristics and combinations of characteristics stated below in the description of the figures and/or shown in the figures alone can be used not only in the specified combination in each case, but also in other combinations or in isolation without departing from the scope of the invention.
Further advantages, characteristics and details of the invention can be seen from the claims, the following description of preferred embodiments and with reference to the drawings.